Cathode-pulsing communication system



Feb. 13, 1951 w s HALSTEAD 2,541,050

CATHODE PULSING COMMUNICATION SYSTEM Filed May 10, 1945 4 Sheets-Sheet 1 FIG. I

I 39 1 omvme MOTOR 4s l 1 g) 1 I43 36, 1 l fi i 1 FIG. 2 I

OUTPUT FROM g moms DETECTOR 5s 'L M HEATER SUPPLY j 5 WILLIAM s. HALSTEAD 39 L/49 INVENTOR 1 441 l :I l l 3 45 i a I 4 L A1 f -1 ATTORNEY CHECKING OR CALLING Feb. 13, 1951 w, s, HALSTEAE 2,541,050

CATHODE PULSING COMMUNICATION SYSTEM Filed May 10, 1945 4 Sheets-Sheet 3 FIG. 6

2I E |2Q-\'9| CLOSED\ 5; I00- D: g .80- 92m g 6O 93 OPEN OPEN O I :c 40- E 0 2O 92 I l I l l I I I 0 l 2- 3 4 5 6 7 8 TIME (IN SECONDS) FIG. 7

' CHECKING OR CHECKING OR Lu EFFECTIVE CALLING EFFECTIVE CALLING 0 '2 *SIGNAL PERIOD- -SIGNAL PERlOD-- l,- 9? r- IQ- I I- ?fm I I CATHODE I 2 I I CURRENT I 6- l I GURVE(FlG.6) I o I I I l g I M I I I 3, 2' II L92 I I I I o I 2 3 4 5 6 7 8 TIME (IN SECONDS) WILL/AM 5. HALSTEAD INVENTOR I ATTORNEY Feb. 13, 1951 W. S. HALSTEAD CATHODE PULSING COMMUNICATION SYSTEM Filed May 10, 1945 FIG. 8

4 Sheets-Sheet 4 1m 6 A.F. SIGNAL GENERATOR H3 H4 I15 I02 IIO K F3 cALL INDICATOR LIGHT TRANSMITTER I08 GALLING J SWITCH [I03 8 1 -|23 I21 I22 PULSING UNIT -Qj]-Io? H6 I04 109 r F hos POWER SUPPLY V I0? F-I "CHECK" SIGNAL E-2 "TALK" SIGNAL F-3 "GALL" SIGNAL FIG. 9

TALK INDICATOR LIGHT "CALLING SIGNAL" CARRIER MODULATED BY "CHECKING SIGNAL" SIGNAL OUTPUT (RELATIVE UNITS) T I I I I32 CARRIER MODULATED BY F-Z "TALKING' SIGNAL" TIME IN SECONDS 'W/LL/AM 5. H/ILSTE'AD INVENTOR Patented Feb. 13, 1951 CATHODE-PULSING COMIVIUNICATION SYSTEM William S; Halstead, Purchase, N. Y., assignor, by mesne assignments, to Farnsworth Research Corporation, a corporation of Indiana Application May 10, 1945, Serial No. 592,993

7 Claims. 1

This invention relates to radio communication systems in which the power consumed by a radio receiver, during stand-by conditions, is reduced in average quantity.

More particularly, this invention relates to radio communication systems in which the receiver is cyclically shifted from a first condition wherein it is fully sensitive to received signals, to a second condition wherein a relatively very small amount of power is consumed thereby. The a cyclic shifting is performed at such a rate, relative to the emission of calling signals from the transmitter, that such calling signals, persisting over an appreciable length of time, i. e. for three or four cycles, will actuate the receiver during some portion of its cyclic changes, when the receiver is operating at a level of normal sensitivity.

In further developed forms of this invention, there are included arrangements whereby the receipt of a calling signal will automatically cause an interruption of the cyclic power pulsing at the receiver, thereby maintaining the receiver in condition for receiving such signal continuously, and will warn the operator to switch manually to a non-pulsing condition, in order to receive a communication immediately following such calling signal.

In another development of this invention, the emission of a carrier wave by the transmitter, preparatory to the transmission of an actual message, sufiices to cause automatic suspension of the power cycling operation at the receiver and causes such receiver to remain in a steady state of sensitivity, suitable for the reception of a subsequent communication upon such carrier wave.

In other developments of this invention, a plurality of characteristic audio frequency modulations emitted by the transmitter are used, respectively to suspend power pulsing at the receiver, to give one or more alarm signals to the operator of the receiver, or to indicate to such operator that both the transmitter and the receiver are properly functioning, or all of these operations may be accomplished simultaneously.

One object of this invention is to economize the secondary type, and, due to the relatively limited amount of energy available from such cells, it is eigtremely desirable that the power withdrawn from-the cells be minimized, in order to extend, to the greatestpossible-degree,the num ber oi hours or days over which such; w llg talkie may be used, before it becomes necessary to replace or to recharge the batteries thereof.

Another object of this invention is to provide a radio receiver in which the cathode heatin current is periodically reduced to a very low value during stand-by operation of the receiver, while yet keeping the receiver in condition to respond to a calling signal, but in which receiver such cathode heating current is steadily maintained at the normal value thereof during periods of actual message reception.

Yet another object of this invention is to economize the use of anode supply current in a radio receiver durin conditions of stand-by reception, by periodically reducing the temperature and consequent electronic emission of the cathodes of the various tubes in such receiver, thus simultaneously reducing or substantially eliminating the flow of anode current through these tubes.

Still another object of this invention is to provide a radio receiver in which the cathode power, and optionally the anode power are supplied to the receiver tubes only intermittently, at normal operating values thereof, until the momentary reception of the carrier wave upon which signal modulations are about to be transmitted, whereupon the receiver mechanism causing the intermittent power supply is automatically rendered inefiective and the receiver is supplied continuously with the full amount of normal operating power.

Another purpose of this invention is to provide a radio communication system in which the receiver, during stand-by periods, is fed with pulsing power and in which the transmitter, during periods of message transmission, emits a controlling or holding modulation of predetermined frequency, which modulation, upon the reception thereof, actuates a relay mechanism located at the receiver, this causing the pulsing power supply to be substituted by a continuous power supply suitable for message reception.

Yet another purpose of this invention is to provide a radio communication system in which the receiver normally is supplied with pulsing power and the transmitter is arranged to emit a modulation of predetermined frequency as a calling signal, such modulation being received and utilized to give visual or audible warning to the operator of the receiver, in order that such operator may manually alter the power supply to the receiver from a pulsing supply to a continuous supply, in order to allow message reception, or that he may take any other steps desired, to assure proper message reception.

Still another purpose of this invention is to provide a, radio transmission system in which both transmitter and receiver are energized sufficiently .to,,, perform the respective functions th reo 9 571 ntermi te y r in s. nd. in

3 which a special checking signal is periodically or continuously emitted; by the transmitterand utilizedby thereceiver'to actuate thereat-a warning light or other suitable arrangement indicating to the operator of the receiver that the entire communication system is.- functioning prop-.--

erly, notwithstanding the fact that no actual.

message is being transmitted overthe system at the time. i

A yet further object of this-invention is to provide a radio receiver in which the cathode heating circuit is periodically andi intermittently broken, so that the heating current is broken up.

the receiver is in a condition to receive such checking or calling signals, thatvacall orchecking' signal from the transmitter will notfail to "be received, provided-only that such signal persist more thantwo or three complete cycles, notwithstanding the absence-of accurate synchronism b'etween the-respective transmitting and receiving cycles.

'An additional. purpose of this invention is to provide a' radiotransmitter inwhich currents of 'several different predetermined audiomodulation frequencies are employed, in addition to voice modulation frequencies, and in which such frequencies as.- are' employed for calling and check- ;ing' purposes are: transmitted: in intermittent pulses, while anothercurrentof a frequency acting tomaintaina receiver. operative, i. e'. a hold? ing frequency, is transmitted continuouslydun the time. that voice; modulated frequencies are being: transmitted.

This invention attains the above'rectified purposes by providing inter alia, a switching. device at: the radio receiver which. alternately shifts the cathode; heating current from a normalvalue to. a value suificient tomaintain the cathode temperature at apoint justbelow electron-emis sive' temperature, which point is herein termed the: temperature of" incipient emission, andxback again to: a normal heating value.-

. Onex otth'e advantages-of this invention is that it: may be: employed. at unattended carrier currentrepeater" stations andthe like, to economize in the use of: power-thereat, thereby'en.-

sibling the batteries: located. thereat to be made smaller and more compactor, alternatively, to berechargedi or replaced atless frequent intervals, and consequently reducing upkeep expenses arising from necessary inspection and attendance at such normally unattended stations.

Further advantages of this: invention will be apparent from the following specifications and hereunto appended drawings, where;

' Fig; 1 is a schematic diagram showing cathode pulsing as applied to a receiving circuit.

Fig; 2' schematically illustrates a portion of a receiver in which cathode pulsing is controlled by-a received carrier.

v Fig. 3 is a block diagram of a receiveroutput stage showing the cathodep'ulsing controlled by a; holding tone.

me;- 4 illustrates; in blockui'as'ram. a receiver output stage controlling both a cathode pulsing unit: and. call indicators by a single: tone frequency.

Fig. 5 represents, in block diagram, 2, receiver output stage embodying operation of cathode pulsing, calL indicators, and a checking indixcator, all by separate tone frequencies.

Fig. 6 is a graph showing one typical receiver cathode pulsing time sequence.

Fig: 7'is:a graphshowing superimposed transmitter pulsing andreceiver cathode pulsing se- 1 quences.

Fig. 8 is a block diagram portraying a transmitter and associated controls for use with a cathode pulsed receiver.

Fig. 9 isa graph illustrating thezrelativeamplitudes of' receiver output: signals; of differin' types;

Fig. .1 schematically illustrates: a. snper regenerative' receiver and. associated. audio; amplifiers.

Antenna I I is connected to inductancev I'Z, which latter is inductivelycoupled to;the1tuning;.cir:- cuit, consisting" of inductance. i4, and variable capacitorl-G;

The signal output. of. detector tube I5? passes through. the radio frequency chokes. 19 and" 20 to the. primary) winding .22 of interstage: tTaRS-.-'

former 23. Volume" control is accomplished-by shunting potentiometer 26 across secondary winding 24 of interstage transformer 2-3;. the variable arm connecting-to control gridv of audio frequency amplifier tube 28.v -Audio tube 28 is resistance coupled through: coupling; capacitor 2.9. to output tube 3.0, which latter energizes speaker 3.2 by means of' output transformer 33. Regeneration is controlledby'varying the plate voltage applied to tube lithrough-potentiometer The negative terminals of both platesupply battery 3.5 and cathode supply battery 36-are.con-

nected to ground through. on-and-off. switch 31.

The positive terminal of the plate supply battery connects to the plate circuits of tubes |5,128',

.and and the'positive terminal of cathode bat.-

Any suitable driving mechanism, such as aspring driven motor, 43, is. connected through shaft to cam 45. The lever or stem.43' illustrates conventional means for rewinding the motor spring mechanism. If it is electrically driven, suitable current feeding means are-substituted for element 43'. When the cam operated cathode pulsing control-switch. is in openapcfiii) sition; as shown in Fig; l, the. series cathode re sistor 39 will maintain. the desired. lower level and 3B,- andallowing these tubes to function at full sensitivity, thereby permittin 'any signals, which may be picked up by the receiver, to be heard from speaker 32.

When the" operator hears an incoming signal during the periodic or momentary" time interval that thecathodesandt'ubes are functioniu'gnonmany, he may" change over: fromintermittent listening operation to steady listening operation, by the manual closing of switch 50. When switch 50 is closed, it shorts out the series cathode resistor 39, and consequently restores full or normal cathode current to all the tubes, thereby causing the receiver to assume the normal or steady state for reception and reproduction of signals.

When the operator no longer desires to have the receiver continue to function in a steady state of reception, with the consequent higher current "drainage, he may cause it to revert to the intermittent and more economical state of operation by opening switch 50, which operation will effectively reinsert the series cathode resistor 39 and the cam operated cathode pulsing control switch 46 into the battery circuit, or circuit from any other suitable source of cathode heating power.

The speed of driving motor 43 and the shape of cam 45 may be such that the cathodes open ate at their normal or full operating current for a -time interval of one second, and at reduced current for the following two seconds, repeating this cycle as long as motor 43 is operating, but such particular time values are purely illustrative, and any other suitable values for the time intervals may be employed.

' In certain instances the motor 43 may consist of an electrically driven motor combined with reduction gears or of any other suitable driving cially desirable in the case wher batteries are employed as sources of power. In such cases, the tubes of the receivers are usually provided with directly heated cathodes, of the filamentary type. Such type of cathode responds thermally very rapidly to changes in the current passing therethrough, so that upon the initiation of current flow from battery 36, as brought about by the closing of contacts 41 and 48, the restoration of normal cathode emission in the various valves takes place in a very short space of time,

for example usually within a small fraotion'of a second.

In order to secure the mode of operation just described, it is suflicient to omit resistor 39 from the pulsing device shown in Fig. 1'. an arrangement, contacts 41 and 48 act to close With such and open the filament circuit in periodic manner,

-so that the current pulses thereby produced are "of substantially square wave form.

' When employing the modification just de-' scribed, the actual breaking of the filament circuit, which takes place when contacts 41 and 48 separate from one another, frequently gives rise to a momentary pulse, which actuates loudspeaker 32, via transformer 33, thus causing a click to be heard from the loudspeaker.

The presence of a succession of rapid clicks, may in some instances be found undesirable, for example where a standby receiver must be kept as silent as possible. It is possible to minimize or completely to avoid the occurrence of these clicks, by shunting a condenser of suitably high capacity across con" "'tac'ts 41 and 48, i. e. so that such condenser will occupy the same position as thatshown in the drawing for resistor39z Whenco'r tacts' 41 and 48 separate, such condenser will tend to prevent too rapid extinction of cathode heating current flow, by reason of the fact that the current flow will continue, in constantly diminishing intensity, until such condenser is charged to a potential substantially equal to that of battery 36. The current flow through the cathodes will accordingly be stopped, not in an abrupt manner, but in a more gradual fashion, so that the production of undesirable clicks is thereby avoided, or sufliciently reduced in intensity.

Fig. 2 shows a cathode pulsing circuit as controlled by the output of a diode detector of a superheterodyne receiver. Tube 60 is a dual triode in which one triode section functions in an audio amplification stage, coupled through capacitor 59 to output tube 63. The remaining triode of tube 80 functions as a squelch control, while potentiometer 56 acts as a diode load, and also permits regulation of the signal-input volt age to the squelch control triode section of tube 60. Switch BI provides a convenient means of cutting the squelch in or out of the circuit, for purposes well known in the art.

Carrier current signals, have previously been rectified in a diode detector circuit (not shown), and the audio frequency components derived therefrom are applied to the grid of the audio amplifier triode section of tube 60. There are provided means for this carrier signal to perform a dual function in the plate circuit of the same triode, namely, to supply audio excitation to the control grid of power output tube 63, and to actuate relay 52.

The driving motor 43 is connected through shaft 44 to cam 45. The rotation of cam 45 will cause contacts 41 and 48 alternately to open and close at predeterm ned intervals, as described in connection with Fig. 1. In Fig. 2 the making and breaking of contacts 41 and 48 of the cathode pulsing control unit 49 causes series cathode resistor 39 alternately to be inserted, or effectively to be removed from cathode heaters 51. The result is that the receiver is being alternatel'y operated in a state of full sensitivity, and in a non-responsive state. When indirectly heated cathodes are employed, as in Fig. 2, the relative durations of the operating and non-operating periods of pulsing control 49 are selected so as to compensate for the slower heating and cooling action of an indirectly heated cathode type of tube. In this case, substantially no clicks are produced by the make and break, due to the electrical separation of heater and signal circuits.

A carrier operated relay 52 i shunted across plate load resistor 5! in the plate circuit of the effective removal of resistor 39 from the cathode circuit, restore and maintain the receiver at full operating efliciency, in so far as cathode heating is concerned. Thus, there is accomplished, by an electromagnetic relay, the automatic determination of alternate listen and stand by conditions in the receiver. The removal of the carrier signal from the receiver will cause the how of plate current in the amplifier section of tube 68 to decrease, and the subsequent de-en- "ergizin'g of relay 52 causes contacts 53 to open,

thereby placing the receiver in "stand by" con- ,cuency, F3.

l-passes substantially only a predetermined frecuency, F2, which frequency is then fed through rectifier 12, of which the output voltage is used to energize hold-in relay 13. Upon the closing of contacts 34 of hold-in relay 13, the receiver is automatically changed from the intermittent monitoring condition in whichait had been "main- 'tained by cathode pulsing control 49;, to 'a. state of steady reception, until there occurs the-revmoval of carrier borne hold-in or talk signal irequency F2 from the receiver input. Atthis time the hold-in relay releases andopens contacts 7 74. The receiver will then revert to the a state of periodic .monitoring. By employing for F2; .a supersonic irequency, relay 13 will continue to operate without causing interference with the nance of a condition of non-interferencewith the reproduction 20f speech. -'Ihereicne, by urtithing a frequency lower than that which the speaker 32 can efiectively reproduce, separation of the hold-in frequency from the speech frequencies may also effectively be secured.

, Fig. 4 is a diagram illustrating the application of a call light H and signal bell 1:8 to the output of a receiver; Receiver output tube 39 is couplied through output transformer 3.3, both to a loudspeaker 32 and to a band pass filter 19, which latter will. pass a different predetermined ire- The output F3, of band pass 19 is passed through rectifier Y80., and the resultant voltage is applied to relay 8|, thereby causing it to closethe two pairs of contacts, '32 and 83. The closing of. contacts 83 applies potential from battery 84 to the call signal indicator lamp H, and also to the buzzer or bell 78. With the closing of contacts 82, the receiver is temporarily changed from its intermittent monitoring state, in which it had previously been maintained by cathode pulsing control 4 9, "to a condition. of

steady'reception, as long as the call signal, or tone frequency F3, is used to modulate the car-- rier wave. The visual and aural call signal indicaters .1! and 18, may be operated continuously, or may be keyed. in. response to a. predetermined codeor other selective method of applying ire- .quency E3 to the carrier signal at the. transmitter,

A manually'ope-rated switch 59, which is shunted across filament pulsing control unit 419 is used by the operator of the receiver to allow steady reception of the communication or message tollowing the calling signaL. The releasing of manual switch 59 restores the unit to the monitoring.

condition, and. thereby consequently eifects economy of tube and battery life.

Fig. .5 is a diagram in which is shown areceiver 7 output arrangement wherein three separate band-pass filters 19, H and 19 and the respective.

rectifiers 69, 12 and 80 are used to operate the respectivev relays 13 and 85, thus enabling the A receiver outputtube 3.9 is coupled through.

transformer 33 to the input of band passifiltere 10,11 and .19., andalso to loudspeaker 3:2, in panxa-llel with all three filters.

:Band pass filter 19 passes checking modular.- tionfrequency Fl to the rectifier 69. The cinerent outputiromrecti-fier 59 feeds relay 68, thus determining the closing and opening of;

contacts: '68", thereby determining the application of current. from .local battery 68A to therecelving check light 66B, thus causing the. light to glow or to be extinguished according toxthe presence or absence of incoming current modulated at frequency Fl. Checkingfrequencyil is transmitted and received at regulargshort inter uals, and. the consequent periodic glowing and extinguishing of the check/signal indicator lamps :5 6B furnishes. a proof of the integrity of the sys. tem, so that the operator at the receiver may constantly be informed as to the operativeness of both the transmitter and the receiver.

Band pass filter H will pass another modulartion frequency, F2, to the input-of rectifier Eli. The current output from rectifier I2 is then applied to. the winding of hold-in relay 73, thereby causing. contacts 13' to close. The relay contacts 13 are connected in shunt with the cathode pulsing control 49, and the closing of contacts 13" causes the receiver'to change from a state of intermittent monitoring to a state of steady reception, as previously explained in the discussion of Fig. 3..

Band pass filter 1.9 will pass yet a third'modnlation frequency F3, termed the calling. frequency, to the input of rectifier 80. Theoutputcurrent from rectifier 80 is then used to energize relay 85,, this resulting in the closing of relay contacts '85 and in the application of electrical energy from battery 84 to the call indicator light 11 and to the bell 18. This call indicator circuit is especially advantageous; inasmuch. as the transmitter opera-tor, immediately prior to a communication, or at any other desired time can, by mod-ulating the carrier wave with frequency F3, sub

stantiall-y instantly call the'receiver operatorfs attention, both by-audible and by visual means. a

Fig. 6 is a graphic representation showing the pulsing or time control, as applied to the cathode current control of vacuum tubes employed with receiver circuits such as. those shown in Figs. 1., 3, 4' and 5. The vertical scale 91:, of ordinates is graduated in units of the cathode curren-t,.expressed in milli-amperes. The horizontal scale 92, of abscissas, is graduated in units of one second and shows thee-time cycles, Curve 9.9 shows. a

minimum. cathode current of .20 :milliamperes tor the first. two. second interval, after which the cur: rent rises, for the duration of the third interval of one second, to its full or normal value of; 100

milliamperes.- This current time cycle is repeated, corresponding to the rotation of cam,

thus affording cathode pulse control, determined by oiexampleandnotor limitation, r

phic. representation. oi'th interval of one second out of three consecutive.

seconds, and when the transmitter checking or calling period has a duration of three consecutive seconds out of four, that the checking or calling. signal will be received at three out of every four of the intervals during which the receiving tube cathodes are operating at their normal emissive temperatures. Temporal overlap of the transmitter and receiver cycles thus determine proper cooperation, notwithstanding the absence of synchronism between the respective cycles.

- While there have been shown illustrative values for the time cycles at the transmitter and the receiver, it will be apparent that these time cycles may have values different from the precise values shown in Figs. 6 and '7. As long as the length of each transmitted pulse is greater than the length of each interval when the receiver is in a non-sensitive condition, the transmitting pulses must of necessity overlap some interval during which the receiver is operating at full sensitivity, so that a persisting call signal will not fail to be received, provided that this relation of :r

relative pulse duration be maintained.

In both Figures 6 and 7, it has been assumed that the cathodes at the receiver are of the filamentary type, which type heats and cools with relatively great rapidity, so that the electron emission will substantially correspond with the current waves 93 and 93. Actually, due to the thermal lag of the filaments, the vertical portions of curves 93 and 93 will assume a slight angle from the perpendicular, but this factor does not materially alter the operation of the system, as just described.

However, when indirectly heated cathodes of the type shown in Fig. 2, are employed, the phenomenon of thermal lag takes on increasing importance, so that curves 93 and 93 will depart to a considerableextent from a strictly'square wave form, and consequently the length of time during which such cathodes are operating at fully normal emissive temperature will be somewhat reduced. In such cases, it may be found desirable to lengthen out the individual cycles, either at transmitter or at receiver, or at both these points, in order to make certain that coincidence of normal sensitivity at the receiver with the transmission of a calling signal, is secured within the space of a relatively small number of cycles.

Fig. 8 is a block diagram of a combination transmitter and communication controls, consisting of antenna IDI connected to the output stage of transmitter I02 and including pulsing unit I 03, power supply I04, audio frequency multisignal generator I66, microphone I 31, calling' switch I08, and press-to-talk switch I09. Also there are shown call indicator light IIB, talk indicator light III, signal light battery H2 and modulation amplitude controls H3, H4 and H5. Pul'sing unit I03 is connected to the circuit of transmitter I02 in such a way that the pulsing 10 unit automatically controls the operation of transmitter I02 by placing it in an operative condition for a time cycle of three consecutive seconds followed by a one second inoperative period, as shown in Fig. 7, curve 97, such value being illustrative, and not restricting.

tially identical with the receiver pulse control unit it, or may assume any other suitable form.

During this stand-by period, the pulsed carrier wave is modulated by check frequency Fl, from generator Hit, the degree of modulation being controlled by modulation amplitude control H4 which regulates the output, at frequency Fl, from the audio frequency generator IE6, the signal then passing through contacts HE and Ill of switch I39, which latter in turn applies the signal to transmitter 12. Instead of pulsing the carrier, it is possible alternatively to transmit a continuous carrier and merely to pulse the modulating check frequency FI The checking signal maybe of any predetermined suitable frequency, preferably 3000 cycles or higher, which is above the normal band of communication speech frequency, or a subsonic frequency may be used, but any other frequency may be employed, a frequency within the normal speech range yielding a loudspeaker checking signal additionally to the visual signal.

Talk frequency F2, or the receiver hold in signal, may be, for example, a 50-cycle tone or a supersonic frequency derived from audio signal generator I 86 and connected through modulation amplitude control H3 to contact H3 of pressto-talk switch 39. When switch I09 is closed, the movable contact III makes connection with contact H8 thereby causing the application of check signal frequency F2 to the transmitter I62, for purposes of modulation. Simultaneously, the depressing of switch I89 removes the pulsing unit let from the circuit and places the transmitter 32 in full operation. At the same time, the closing of contacts H9 and I20 of switch I09 causes the talk indicating light III to glow, by applying to it current from battery H2, thus giving the transmitter operator visual indication that he may then proceed to employ voice modulation. Frequency F2 and speech frequencies are applied simultaneously to the transmitter, the purpose of employing the F2 frequency being to operate the hold-in relay I3, (Fig. 3), and to maintain the receiver in normal listening condition.

, Still referring to Fig. 8, call frequency F3 supplied by the audio frequency generator W6 is fed to modulation amplitude control i it and then to movable contact I2I of calling switch I08. When switch IEBB is placed in the call position, contact I2] is closed against contact I22, thus causing the application of the calling frequency F3 from gen erator I36 to the transmitter, for the purpose of tone modulation. The call frequency may be, for example, a 1000 cycle tone, which will be heard at the receiving position through loudspeaker 32, (Figs. 4 and 5), simultaneously actuating relay 85, this actuation resulting in call light Ti and bell or buzzer It being placed in operation. Any other suitable frequency may be used for frequency F3.

- The depressing of call switch Hi8 (Fig. 8) also removes the pulsing unit I03 from the circuit, and by the closing of contacts I23 the power supply IE4 is placed in normal operation. At the same time, call indicator light I I0 is excited from the batteryv I I2, due to the simultaneous closing of switch contacts I24.

Mech anism for securing this result may be substan Fig; 9' portrays the'tnreeuisunctcurves-er quenc1es-F|-,--F2, and F3yWhereFl i's the' c a g signal frequencm F2 is the talking signal ire queasy, and F3 is the calling signal iredu nay, Fig.0 shows the various irequencies'asbeing of= different signal intensity, the frequency strength being controlled by themodulationamplitude coir trtils 1-1-4, I I3, and 5 (Fig. 8),- for the frequencies Ff, E2; and F3, respectively.

{Three curves, I31, +32 and |-3-3 represent-the ifi'equenciesFl, F2, and F3 respectively. While the three curves are shownas having certain remtive amplitudes, differentfrom one another, it-is td'be understood that this is" merely an arbiti' ry showing and that the relative amplitudes may have any other values, consistent with the ar ticular requirements of the operator and su'f ficient to satisfy any operational requirement.

Curve Isl-represents a relative receiver output, when the receiver istunedconstafitly to a carrier wave modulated by the checking signal-frequency Fl; the checking signal pulsebeingof threesec- 'ondsduration. Curve I32 represents the relative receiver output when tuned to acarrier wave modulated by the talking signal frequency F2,

the continuous wave being maintained for the durationof thecommun'icatio'n which is repres'iited by onee'ndof the curve extending beyond the duration ra-11y heretofore referredto pulse period: Curve I33 represents the relative receiver output when turned to a carrier wave mod-- mated by a calling signal frequency F3, the call ing signal pulse being of three seconds ('iujration.- All the values just given are by waybf exampleand not of limitation.

I claim:

1. A radio receiver including means ior "periodically reducing the sensitivity thereof, signal demodulation means, frequency selective means fed from said demodulation means and re'spon sive to a predetermined signaling frequency, a i'e'ctifier fed from saidfrequencyselective means, a relay fed from said re'ctifierand having two sets of contacts; a warning signal coupled to one of said sets of contacts, said other set er contacts being coupled to said'fir'st means and acting to Suspend the efie'c'tive action of said periodic sensitivity reducing -means, as long as said predetermined signaling frequency is being received, whereby said warning signal functions lifiintrrllptediy while said signaling? rre uencyis being received. 2'. In a radio receiver having atubewith an electrically heated cathodefa current saving dc vice operative during standby recepucnsaid aei vice including, connected between "a sourceof cathode heating current and said cathode, a re sli'storlarge enough to reduce 'said current to Value S'uficint tb heat Saidtlibe 'to'the point or incipient emission only, a normally operative make and break switch cyclically short circuitiiig said resistor so as to restore saiah aungene'rgy to substantially full value suitcase-for norfnal emission, and a switch serving when closed steadiii! to short out said resistor, whereby steady operation of said receiver during message reception is secured by closure of said'switch.

I 3. A radio signaling system, including at the transmitter meansfor emitting a-carrier', means f'ormodulating said carrier by three difierent tone frequencies and also by speech frequency, and'ii'rcl'uding at the receiver an electronic'tub'e and means for supplying heating current thereto, means normally periodically reducing said tube heating to: avaiue 'suflicient'to neat tube only to incipient emission, reiaymeansaci tuated by the first received tone iirequency w nullify theeffect or said periodic currentreducing means,- signaling means actuated by the second received tone-frequency to indicate: theoperation of said transmitter and signaling-means actuated. by the third received tone frequency to: give alarm, thereby indicating that said transmitter" is ready to bemodulated at speech irequency,

4. A- radio receivercomprising a circuit inane ing a plurality of electron tubes, a cathode ener gi-zing source, an impedance eonneet'e'd to said source and the cathodes of Said tubes for con trolling the currentflow therein, a switch con 'nected to said impedance, means-for pe'iiod-icalI-yoperating said switch to change the current in said cathodes, asec'on-d 's'vvitchc'onnected across said impedance-,and a carrier-current responsive device connected "to said circuit for-operating said second switch.

5. A radio receiver comprising a circuit includ ing a plurality of electron tubes, a cathode liner"- gi'zing source, an impedance connected to id source and the cathodes of said tubes for limit= ing the current flow therein, a-switch-c'onnected to said impedance, means for periodicallyoprat ing said switch to change the current 'in said cathodes, a second switch connected across said impedance, and a relay connected tosai'd circuit and responsive to received signals for operating said second switch. Q

6. A radio receiver compri'singa circuit includ ing a plurality of electron tubes, at cathodeener gizing source, an impedance connected in series with said souiceand the cathodes of said tubes for limiting the current flow therein, a switch connected 'acrosssaid' impedance, 2; mctcr open ated cam for periodically'closing said-switch'to increase the current in saidcathodes, a-secoiid switch connected acrosss'aid impedance, and a relay connectedto' said circuit andre'spoii'siv'e to received signals 'forclosing" said secondswitch. 7. A radio receiver comprising a circuitindlud= ing a plurality "of el'ectron'tubes, a, cathode mar-- gizing" source, a resistor connectedin series with said source and the cathodes'oi said "tubes'ior limiting the current new therein, a switchfc'onnected acrosssaidresistor, a motor-operated earn for periodically "closing said switch"to increase the current in said cathodes, "a second switt'ah connected. across said resistor, and an electromagnetic relay connected to said circuit andra sponsiv'e toreceived signals for closing said sec:- ond' switch. I, p p

REFERENCES CITED The following. references are for record in 1t me or this patent UNITED STATES PATENTS 

